1. Field of the Invention
The present invention relates to a paint coating apparatus and a magnetic recording medium and in particular, to a paint coating apparatus having a die head for simultaneously forming at least two paint layers on a surface of a support body and a magnetic recording medium using the same.
2. Description of the Prior Art
Conventionally, a magnetic recording medium such as an audio tape and a video tape is formed by painting a magnetic paint on a flexible support body such as polyester. The magnetic paint is prepared by a magnetic powder, binder, dispersing agent, and lubricant which are dispersed into and kneaded with an organic solvent. This is a so-called paint type magnetic recording medium. The paint is applied onto the support body by the roll coat method such as gravure roll method and reverse roll method. Recently, the extrusion method has been implemented in practice, enabling to obtain more uniform thin film coating. In the paint coating apparatus employing this extrusion method, it has been cleared that the paint coating quality is greatly affected by the die head tip configuration which is brought into abutment with the flexible support body.
FIG. 1 is an enlarged schematic view of the die head tip end, where a front lip 2a, a first paint discharge slit 2b, an intermediate lip 2c, a second paint discharge slit 2d, and a rear lip 2e are arranged in this order from the upstream. At least a plurality of paint liquids are simultaneously applied to the support body 1 so as to form layers. that is, in the example of FIG. 1, a lower paint is discharged with a predetermined pressure from the first paint discharge slit 2b and almost simultaneously with this, an upper paint is discharged with a predetermined pressure from the second paint discharge slit 2d, thus forming two paint layers almost simultaneously. In a paint application apparatus using such a die head 2, even if the lower paint and the upper paint have different viscosities, it is necessary to obtain a desired thickness of the lower layer and a desired thickness of the upper layer. Especially recently, a shorter wave of recording is desired in the magnetic recording medium, which requires a further uniform and thin magnetic layer, increasing the importance of the tip configuration of the die head 2.
In the conventionally used die head 2, for example, if the lower paint has a lower viscosity than the upper paint, the upper paint forms a thicker film, and it becomes difficult to obtain a desired thickness. The upper paint and the lower paint usually have a viscosity of 1000 to 2000 cps in the B-type viscosity meter and there is no big difference between them. However, when the binder contained in the paint is changed, the lower paint may have a lower viscosity by 50 to 400 cps.
FIG. 2 is a graph showing a paint pressure distribution between the front lip 2a and the rear lip 2e when the viscosity is 1000 to 2000 cps by the B-type viscosity meter. FIG. 3 is a graph showing the paint pressure distribution between the front lip 2a and the rear lip 2e when the upper paint has a viscosity of 1000 to 2000 cps measured by the B-type viscosity meter and the lower paint has a viscosity lower than 400 cps measured by the B-type viscosity meter.
As is clear from comparison between FIG. 2 and FIG. 3, in contrast to case when both of the upper paint and the lower paint have viscosity from 1000 to 2000 cps, when the upper paint has viscosity of 1000 to 2000 cps but the lower paint has viscosity lower than 400 cps, the coating pressure of the lower paint from the front lip 2a to the intermediate lip 2c is lowered and the coating pressure of the upper paint from the second paint discharge slit 2d to the rear lip 2e is increased.
In this case, thought the cause is unknown, the upper paints forms a paint film thicker by 20 to 30% even when the paint supply rate to the die head 2 is maintained constant. That is, as the lower paint has a lower viscosity, the upper paint tends to form a thicker film. This problem should be solved for obtaining a uniform thickness of the magnetic layer.